Optimum ignition conditions for gas turbine engines are subject to variation between starts due to a variety of factors, e.g. ambient temperature, gas turbine temperatures, fuel calorific value, fuel content, pressures, repeatability of fuel and air delivery systems, etc. In a typical start system for a gas turbine engine one uses an auxiliary drive unit for driving the turbine and a control unit providing a start sequence in which turbine speed and fuel delivery are coordinated to provide a fuel/air mixture at an ignition device allowing a successful ignition.
In a typical start sequence, the speed of the gas turbine, which is during start driven by an auxiliary motor, and/or the fuel flow to the combustion system are progressively increased over a set period of time, the so called light-up window. The length of the light-up window is a function of the range of engine speeds at which starting is most likely to occur, typically between 5% and 20% of the rated engine speed and the accumulation rate of fuel in the combustor. During the light-up window a number of ignition opportunities appears, the actual number of which depends on the number of sparks that can be delivered per second by the igniter of the gas turbine engine and the length (duration) of the light-up window. Therefore one likes to have the light-up window as long as possible. However, the length of the light-up window is delimited by a number of factors. If, e.g. the turbine is accelerated too quickly the fuel injection system will not have enough time to provide a sufficient amount of fuel before the window of engine speeds at which starting is most likely to occur is exceeded. On the other hand, if the turbine is accelerated too slowly, it may happen that an amount of fuel inside the combustor is reached which could be dangerous to the engine while the turbine speed has still not reached the maximum speed within the light-up window. However, for example the acceleration rate of the turbine depends on the ambient conditions. On a cold day, a battery driven starter motor may not be capable of accelerating the engine quickly due to possible low power supply. On the other hand, on a very hot day, the same motor with the same battery may be capable of accelerating the engine very quickly. To cope with the mentioned limitations a compromise is typically required between maximizing the light-up window to cover for wide variations in the actual optimum window and minimizing the variation rate to increase the number of ignition opportunities (sparks) during the actual optimum window, without establishing a potentially dangerous fuel amount inside the combustion system during the light-up window.
Typical start sequences for gas turbine engines are, e.g. described in U.S. Pat. No. 5,844,383, U.S. Pat. No. 5,907,949, where a temporary increase in fuel flow during start to enhance the heat release is described, U.S. Pat. No. 4,464,895, where a pulsating (modulated) liquid fuel flow which improves the atomization and thereby the ignition capability is described and in RU 2078971, where the fuel flow is varied in relation to the operating condition for the gas turbine before start.
If a successful ignition cannot be detected after the light-up window, the gas turbine is usually shut down due to a so called “no-light-up trip”.